1. Field of the Invention
This invention relates to the recovery of gold and silver from an aqueous solution containing cyanic compounds of valuable metals. More specifically, it relates to a method of separating and recovering gold and silver from an aqueous solution containing free cyan and also containing cyanic compounds of valuable metals including small amounts of gold and silver.
2. Description of the Prior Arts
Heretofore, it has been proposed to recover gold and silver from an aqueous solution containing gold and silver as cyanic compounds, for example, by contacting the aqueous solution with an excess of oxygen and simultaneously irradiating it with radiation at a wavelength of 20-600 nm to precipitate gold and silver (Japanese Patent Publication No. 31240/1983, published on July 5, 1983); or by passing the aqueous solution through an ion exchange resin bed to adsorb gold, silver, etc., (Japanese Patent Publication No. 15190/1983, published on Mar. 24, 1983) and so forth.
However, the former method was such that the temperature was preferably a high temperature such as 90.degree.-100.degree. C. and the aqueous solution was exposed as a thin layer or as fine droplets to oxygen and radiation of a wavelength of 20-600 nm for 40 hours or longer, preferably for 100 hours or longer (Japanese Patent Application Laid-Open No. 51068/1975, laid-open on May 7, 1975), and thus had the disadvantage of requiring the use of large equipment. In addition, this method was inefficient. The latter method was also disadvantageous in that the ion exchange resin after treatment could not be regenerated and thus the cost was increased, and in addition it had the additional drawbacks of weak adsorptivity, a small liquid volume which could be treated and a great difficulty in reducing the content of the resultant solution after treatment to 0.1 mg/l or less of Au.
Against this background the present inventors proposed in Japanese Patent Publication No. 22530/1983 an improved method of recovering gold and silver from an aqueous solution containing cyanic compounds of valuable metals. According to this method a hypochlorite is added to the aqueous solution until its oxidation-reduction potential indicates+350 mV or higher, mineral acid together with a small amount of active carbon is added to this aqueous solution to make the pH 5.0 or less, the resulting precipitates are separated, and the remaining aqueous solution is passed through an active carbon bed which has been pretreated with mineral acid, thereby adsorbing the valuable substances to the active carbon. This method has enabled the recovery of almost all the gold, silver and other metals from the aqueous solution in which a large amount of alkali cyanide (hereinafter referred to as free cyan) and high concentrations of cyanic compounds of Au, Ag, Cu, Ni, Co, Zn, etc., are co-present.